Platelet-derived growth factor (PDGF) is a connective tissue mitogen which may play a role in the physiology of wound healing in vivo. Addition of PDGF to growth arrested 3T3 cells results in a prompt (within 40 ft.) induction of one or several specific poly (A)+ mRNAs. Induction of these mRNAs is not a mere consequence of cell growth. Rather a variety of data suggest that these mRNAs (or their translation products) function as stable, intracellular mediators of the growth response to PDGF. My laboratory has cloned and isolated cDNA probes directed to PDGF-inducible mRNA sequences within Balb/c-3T3 cells. This application focuses on the regulation of these mRNAs and function of their translation products in the mitogenic response. Manual microinjection procedures and recombinant DNA technology will be applied towards 3 broad objectives. The first objective is molecular characterization of PDGF-inducible gene sequences. "Hybridization selection" and "translation arrest" techniques will identify the proteins which correspond to each PDGF-inducible cDNA probe. The structural genes corresponding to our cDNA probes will be isolated from a cloned genomic library of Balb/c-3T3 cell DNA. The second objective is to establish the function of PDGF-inducible gene products in the mitogenic response of 3T3 cells. This objective will be approached i. by transfection of the cloned PDGF-inducible structural genes into normal 3T3 cells and ii. by manual microinjection of hybridization-selected mRNA into quiescent 3T3 cells. The third objective is to determine whether these gene sequences function as a common intracellular mediator of the growth response in other proliferating systems. Molecular hybridization techniques will be used to screen for expression of PDGF-inducible gene sequences within epithelioid cells and within connective tissue tumors. Additionally, tumor promotors and other mitogenic agents will be screened for their ability to modulate expression of these gene sequences.